The present invention relates to method and apparatus for evaluating the quality of resistance welds, in particular, in spot welding.
Whereas resistance welding, particularly, spot welding has been applied for various products using steel plates, there is a tendency toward increasing welding defects in recent years. That is, because mild steel plates have commonly been materials to be welded hitherto, inferior welding current would scarcely occur so that the welding quality could be maintained relatively stable by controlling the welding conditions constant. However, as large quantities of galvanized steel plates and high tension steel plates have begun to be used in place of mild steel plates, the occurrence of weld defects have been increasing. From such a background, there has been a desired earnestly a method and an apparatus operative for not merely monitoring the welding conditions but also allowing the welding quality to be monitored with high precision.
With regard to this problem, there have been developed a variety of welding quality monitoring systems with the aim of judging, after the completion of a welding process, whether the welding result is good or poor. A judgment as to whether a welding result is good or poor, if possible, allows the result to be reflected on the next-time welding. Such welding quality monitoring systems that have been developed so far are exemplified by:
(1) a system in which an inter-chip resistance is determined from welding current and welding voltage and, based on a change pattern of the inter-chip resistance, the goodness or poorness of a welding result is judged, an example being disclosed in Japanese Patent Laid-Open Publication SHO 56-158286; PA0 (2) a system in which an inter-chip voltage is compared with time change of a preset reference voltage and, depending on whether or not the difference between them is within an allowable value, the goodness or poorness is judged, an example being disclosed in Japanese Patent Publication for opposition SHO 59-14312; and those in which an effective component that effectively contributes to heat generation of a weld is extracted from an inter-chip voltage and, based on a time integration value of the effective component, the goodness or poorness of a welding result is judged, an example being disclosed in Japanese Patent Publication for opposition SHO 59-40550 and Japanese Patent Laid-Open Publication SHO 59-61580; PA0 (3) a system in which exothermic temperature is detected and, based on its temperature changing pattern, the goodness or poorness of a welding result is judged, an example being disclosed in Japanese Patent Laid-Open Publication HEI 1-216246; PA0 (4) a system in which ultrasonic waves are transmitted between materials to be welded and, based on the resulting amount of transmission, the goodness or poorness of a welding result is judged, an example being disclosed in Japanese Patent Laid-Open Publication SHO 52-94841; PA0 (5) a system in which a signal derived from an acceleration sensor mounted on an electrode chip is double integrated and the resulting displacement of the electrode chip during a welding process is used, an example being disclosed in Japanese Patent Publication for opposition SHO 60-40955; PA0 (6) a system in which welding current is detected and its upper and lower limit values are monitored with an aim of maintaining welding results constant; PA0 (7) a system in which with a thermal conduction model used, a nugget diameter is calculated by using a computer, an example being disclosed in Sano, "A study on numerical analysis method for welding current path and temperature distribution in spot welding", Master's thesis specialized in welding in Osaka University Postgraduate Course (1979), and Nishiu, "A study on faster quality monitoring with the aid of numerical calculation for resistance spot welding" Master's thesis specialized in welding in Osaka University Postgraduate Course (1991); and the like.
Among these systems, with System (1), when crush or welding current diversion of a chip end portion has occurred or when the material to be welded is galvanized steel plate, it becomes hard to judge the goodness or poorness of a welding result because of non-uniform changing pattern of resistance. With System (2), because of the need of re-setting the judgment conditions for a welding result each time the welding state has undergone a change such as crush of the chip or a change in the plate thickness, it is hard to make an accurate judgment of the goodness or poorness on a practical base. Systems (3) and (4) have difficulties in field-work applicability in terms of the way how to install and mount temperature detecting equipment and ultrasonic transmitting and receiving equipment. System (5) is problematic for practical use because of noise mixing, difficulty in measurement of small displacements, differences in mechanical strength among individual resistance welders and the like which would be involved in applications to welding field work. System (6) is low price in terms of cost, easy to implement and effective for finding power failures, breaking of secondary conductors and the like, but is unable to discriminate quality deteriorations of welds due to lowering of current density such as crush and welding current diversion of the chip end portion.
Further, these various types of conventional welding quality monitoring systems would involve the work of making a preliminary experiment at welding fields for each welding material and then previously determining the relationship between welding quality and discrimination criteria, and besides the discrimination result could only afford a rough judgment as to the goodness or poorness of a weld. System (7), although having a potential capability of resolving the foregoing problems, would take long time to resolve the equation of heat conduction as its greatest defect. Due to this, it would be the case that a welding result cannot be judged until the welding process is completed, so that output control of the welder itself or improvement in welding results could not be achieved.
The object of the present invention is therefore to solve the above problems and to provide method and apparatus for monitoring a quality of resistance welds, capable of monitoring a state of generation of a nugget with general versatility, and of evaluating the quality of welds with reliability.
In order to achieve the above object, there is provided, a method for evaluating quality of a resistance weld, comprising steps of:
detecting instantaneous values of welding current and inter-chip voltage during a current changing period during which instantaneous value of the welding current is changing; PA1 calculating an instantaneous value of an apparent inter-chip dynamic resistance from both detected values; PA1 calculating a rate of change of dynamic resistance instantaneous value of the inter-chip dynamic resistance at an arbitrary time during the current changing period; and PA1 evaluating welding quality by using a change characteristic of the rate of change of dynamic resistance instantaneous value calculated in correspondence to a number of times of occurrence of the current changing period. PA1 welding current measuring means for detecting welding current; PA1 inter-chip voltage detecting means for detecting inter-chip voltage; PA1 inter-chip dynamic resistance calculating means for calculating an apparent inter-chip dynamic resistance from detected values of the measuring means and the detecting means; PA1 rate of change of dynamic resistance instantaneous value calculating means for calculating a rate of change of dynamic resistance instantaneous value of the inter-chip dynamic resistance during a current changing period during which the instantaneous value of the welding current in a welding process is changing; PA1 storage means for storing the inter-chip dynamic resistance and the rate of change of dynamic resistance instantaneous value one after another; and PA1 arithmetic means for performing arithmetic process for judgment of welding quality by using the stored rates of change of dynamic resistance instantaneous value.
In order to achieve the above object, there is also provided an apparatus for evaluating quality of a resistance weld, comprising: